1. Field of the Invention
The present invention relates to a unit injector for an engine.
2. Description of the Related Art
Known in the art (see Japanese Unexamined Utility Model Publication (Kokai) No. 62-64872) is a unit injector provided with a plunger which is driven by an engine, a high pressure fuel chamber filled with fuel which is pressurized by the plunger, a needle positioned coaxially with the plunger and moved in accordance with the fuel pressure in the high pressure fuel chamber to open a valve when the fuel pressure exceeds a predetermined pressure, a bore extending between the plunger and needle in a perpendicular direction with respect to the common axis of the same, a spill valve inserted slidably in the bore to control the spillage of fuel in the high pressure fuel chamber, and a piezoelectric element which moves the spill valve axially relative to the bore and controls the opening and closing of the spill valve, wherein the fuel injection is performed when the spill valve is closed.
Further, there is known (See SAE paper No. 850442) a unit injector provided with a plunger which is driven by an engine, a high pressure fuel chamber filled with fuel which is pressurized by the plunger, a needle positioned coaxially with the plunger and moved in accordance with the fuel pressure in the high pressure fuel chamber, to open a valve when the fuel pressure exceeds a predetermined pressure, a bore extending in parallel with and at a distance from the common axis of the plunger and needle, a spill valve slidably inserted in the bore to control the spillage of fuel in the high pressure fuel chamber, and a piezoelectric element positioned axially to the bore and moving the spill valve axially relative to the bore and controlling the opening and closing the spill valve, wherein the fuel injection is performed when the spill valve is closed.
In unit injectors, however, a powerful drive force is applied to the needle since the fuel pressure in the high pressure fuel chamber becomes as a high as 1500 kg/cm.sup.2 or more, and as a result, the injector housing between the plunger and needle is subjected to a large compressive load and is distorted. Therefore, as disclosed in Japanese Unexamined Utility Model Publication (Kokai) No. 62-64872, if the bore is formed between the plunger and needle, the powerful drive force applied to the plunger will cause distortion of the bore, with the result that the sliding resistance of the spill valve will be increased and the operating response of the spill valve will be deteriorated, and thus the spill valve will not be fully closed.
Conversely, in the unit injector disclosed in SAE paper No. 850542, the bore is not disposed between the plunger and the needle, and thus the powerful force given to the plunger will not cause a distortion of the bore. But in this unit injector, the bore and the piezoelectric element are disposed coaxially, and further, the bore extends in parallel to the common axis of the plunger and needle, and the length of the fuel spill passage connecting the high pressure fuel chamber and the bore is increased. This fuel spill passage is communicated with the high pressure fuel chamber, and therefore, if the fuel spill passage becomes longer, the volume of the high pressure fuel chamber is increased. If the volume of the high pressure fuel chamber is increased, it is difficult to raise the fuel pressure in the high pressure fuel chamber to a high level, and thus it is difficult to ensure a good atomization of the fuel. Further, when the spill valve is opened to stop the fuel injection, the fuel pressure inside the high pressure fuel chamber will not immediately drop, and thus the fuel injection will continue even after the spill valve is opened. This fuel injection occurring after the spill valve is opened has a poor atomization due to the low fuel injection pressure, and therefore, causes smoke and has an adverse effect on the engine output and the fuel consumption rate.